BGP - prefix-based outbound route filtering

Prefix-based outbound route filtering is used so a local router can tell it's peer what routes it should send/filter. This prevents unnecessary resources from being used. There is no sense in a router sending a bunch of route updates, if they are only going to get filtered anyway.

In this example we have EBGP peers R4 and R3:

[R4]---[R3]

R3 is receiving a bunch of routes from R4:

R3#show ip bgp Network Next Hop Metric LocPrf Weight Path
*> 1.0.0.0/24 172.12.34.4 0 400 65000 i
*> 1.0.1.0/24 172.12.34.4 0 400 65000 i
*> 1.0.2.0/24 172.12.34.4 0 400 65000 i
*> 1.0.3.0/24 172.12.34.4 0 400 65000 i
*> 2.0.0.0/24 172.12.34.4 0 400 65000 i
*> 2.0.1.0/24 172.12.34.4 0 400 65000 i
*> 2.0.2.0/24 172.12.34.4 0 400 65000 i
*> 2.0.3.0/24 172.12.34.4 0 400 65000 i
*> 3.3.3.0/24 0.0.0.0 0 32768 i
*> 4.0.0.0/24 172.12.34.4 0 0 400 i
*> 4.0.1.0/24 172.12.34.4 0 0 400 i
*> 4.0.2.0/24 172.12.34.4 0 0 400 i
*> 4.0.3.0/24 172.12.34.4 0 0 400 i

R3 only wants to receive 3 routes:

1.0.0.0/24
2.0.0.0/24
4.0.0.0/24

R3 can create a prefix-list allowing these 3 routes only and advertise this to R4. R4 will use this list as a outbound filter. Let's configure it. First you need enable the advertisement of the orf capability. R3 is the one sending the prefix-list so use the send keyword. R4 is receiving the prefix-list.

R3(config)#router bgp 65003
R3(config-router)#neighbor 172.12.34.4 capability orf prefix-list send

R4(config)#router bgp 400
R4(config-router)#neighbor 172.12.34.3 capability orf prefix-list receive

Now configure the prefix-list and apply it to the neighbor:

R3(config)#ip prefix-list ZERO seq 5 permit 1.0.0.0/24
R3(config)#ip prefix-list ZERO seq 10 permit 2.0.0.0/24
R3(config)#ip prefix-list ZERO seq 15 permit 4.0.0.0/24
R3(config)#router bgp 65003
R3(config-router)#neighbor 172.12.34.4 prefix-list ZERO in

R3#clear ip bgp * soft in prefix-filter

Here is the final result:

R3#show ip bgp
Network Next Hop Metric LocPrf Weight Path
*> 1.0.0.0/24 172.12.34.4 0 400 65000 i
*> 2.0.0.0/24 172.12.34.4 0 400 65000 i
*> 3.3.3.0/24 0.0.0.0 0 32768 i
*> 4.0.0.0/24 172.12.34.4 0 0 400 i

Here are some captures I took in dynamips. The first shows the advertisement of the orf capability. The second shows the actually prefix-list R3 is sending. Wireshark shows this as "route-refresh" message. Pretty cool, eh?



Restrictions:

I used the bgp upgrade-cli command to configure these neighbors in AF mode.
Also, prefix-lists must be used, not ACL or distribute lists

For more info go here:

BGP Prefix-Based Outbound Route Filtering